The present application is a continuation-in-part of U.S. application Ser. No. 194,649, filed Oct. 6, 1980, now U.S. Pat. No. 4,381,327.
The present invention relates to the fabrication of laminations of mica and conductive materials, and in particular to the manufacture of ion generating apparatus incorporating such laminations.
Mica has long been known by those skilled in the art to be a suitable dielectric material for use in many different applications. Mica possesses superior dielectric properties, including a high dielectric constant and good dielectric strength. As a stable, inorganic material mica resists eroding by a number of different substances. Mica may be easily fabricated in thin, uniform dielectric layers with thicknesses of 1 mil and less. When fabricated in these thicknesses, mica is an extremely sturdy, durable material.
A particularly common application for mica is as the dielectric component of capacitors. Mica capacitors are normally constructed by "silvering"--that is by printing electrodes onto blades of mica, usually by means of a silk screen process. The silver is applied to the mica in a solution, and the solvent evaporated by firing the composite in an oven. This fabrication technique provides a good connection between mica and electrode, and allows a compact design by avoiding thick blades or foils. Because of the delicate nature of the electrodes created with this process, it is necessary to completely encapsulate the mica-electrode laminate to protect the electrodes from environmental influences.
In certain utilizations, however, it is necessary to directly expose the mica dielectric and electrode material to air. One such utilization is shown in commonly assigned U.S. Pat. No. 4,155,093, which discloses apparatus for generating ions in air. With reference to the prior art sectional view of FIG. 1, the ion generator 10 comprises two conducting electrodes 12 and 13 separated by a dielectric layer 11. When a high frequency electrical field is supplied between these electrodes by source 14, a pool of negative and positive ions is generated in the areas of proximity of the apertured electrode 13 and the surface of the mica. Thus, in FIG. 1, an air gap breakdown occurs relative to a region 11-r of dielectric 11, creating an ion pool in hole 13-h which is formed in electrode 13. This is attributable to an atmospheric "glow discharge", which may be contrasted to the more common "arc discharge" in the lower density of excited atoms, and hence lower currents. An advantageous design of a glow discharge ion generator, such as that of the present invention, produces a high percentage of usable ions and hence surprisingly high ion output current densities.
Ions generated by the device 10 of FIG. 1 may be used, for example, to create an electrostatic latent image on a dielectric member 100 with a conducting backing 105. When a switch 18 is switched to position X and grounded as shown, the electrode 105 is also at ground potential and little or no electric field is present in the region between the ion generator 10 and the dielectric member 100. However, when switch 18 is switched to position Y, at which the potential of the source 17 is applied to the electrode 13, this provides an electric field between the ion reservoir 13-h and the counterelectrode of dielectric member 100. Ions of a given polarity (in the generator of FIG. 1, negative ions) are extracted from the air gap breakdown region and charge the surface of the dielectric member 100. The rate of charging the dielectric surface may be expressed as a given ion current. Although this patent discloses the geometry of applicant's preferred embodiment, it does not disclose the use of mica for the dielectric 11, nor a method of fabricating such an ion generator.
FIG. 2 gives a sectional view of a three-electrode version 10' of the ion generator of FIG. 1, of a type generally described in U.S. Pat. No. 4,160,257, commonly assigned with the present application. In addition to the elements already described, ion generator 10' includes a "screen" electrode 52 which is separated from electrode 13 by a dielectric spacer 51. Screen electrode 52 includes an aperture 53 which is aligned with the aperture 13-h. Dielectric spacer layer 51 is apertured at 55 to permit extraction of ions; aperture 55 is desirably considerably wider than aperture 13-h to avoid wall charging effects. The screen electrode 52 is subjected to a potential 54 which influences the extraction of ions from aperture 13-h. As explained in U.S. Pat. No. 4,160,257, the screen potential 54 isolates any potential on the dielectric surface 100 from the ion generator, thereby preventing accidental image erasure. The screen electrode furthermore provides an electrostatic lensing effect which may be used to control the size and shape of the latent electrostatic image created on dielectric 100.
The ion generators shown in FIGS. 1 and 2 require exposure of the dielectric 11 and the apertured electrode 13 to air. In employing mica as the dielectric, it has been found that laminates fabricated by silvering the electrodes are unable to withstand the incursion of materials, such as ozone and nitric acids, which are produced as normal byproducts of the ion generation process. On the other hand, traditional methods of laminating thicker layers of conducting foils, such as bonding the layers with thermoset adhesives, present the problem that mica is easily delaminated, i.e. cleaved into layers. This might happen at elevated temperatures, or due to the presence of atmospheric moisture.
Although known encapsulation techniques for mica capacitors protect against delamination due to moisture, these techniques are unsuitable for applications which require exposure to air of the conductive material as well as the dielectric. The construction of an externally exposed mica-foil laminate by traditional methods will result in a structure which will tend to deteriorate easily and have only a very short service life. A laminate of the type illustrated in FIGS. 1 and 2 must withstand high peak voltage radio frequency signals, on the order of kilovolts. It is furthermore necessary that this laminate withstand elevated temperatures characteristic of such high voltage RF potentials. Applicant has discovered that the use of thermoplastic adhesive layers 33, 37 (FIG. 2) to bond the various layers of ion generator 10' meets these various criteria.
A June, 1975 article by William J. O'Malley in Adhesives Age magazine, "Silicone Pressure-Sensitive Adhesives for Flexible Printed Circuits", discloses a technique for fabricating flexible printed circuit boards using organopolysiloxane pressure sensitive adhesives. Silicone pressure sensitive adhesives are recommended for this application due to their chemical properties, stability at elevated temperatures, flexibility, and high bonding strength over a broad temperature range. The adhesives also resist heat applied at high relative humidities. This reference does not contemplate the use of extremely thin adhesive layers, however, and in fact indicates unacceptably low peel strength for layers less than 1 mil thick. The laminates disclosed by O'Malley are not well suited to the design of atmospheric ion generators, in that they would provide unacceptably low ion output current densities.
Accordingly, it is a principal object of the invention to provide a method of fabricating durable mica-electrode laminates. A related object is that the laminates of the invention resist delamination due to moisture, and erosion due to ozone, nitric acid, and other substances. The laminates of the invention should be suitable for the generation of ions in air.
Another object of the invention is the achievement of a mica-conductor laminate which exposes the various layers to air. A related object is the avoidance of delamination due to atmospheric moisture and other environmental substances.
Yet another object of the invention is the fabrication of a mica-electrode laminate which is physically stable over a wide range of temperatures. A related object is the achievement of an ion generator which can carry high peak voltage RF signals over a long service life.
A further object of the invention is the fabrication of ion generators which provide satisfactory ion output currents without requiring high drive voltages. A related object is the achievement of an efficient, economical ion generator. It is furthermore desirable to maintain reliable ion current outputs at a plurality of ion generation sites, over the service life of the ion generator.